Hydraulically-latched directional control valves are known for use in sub-sea operations to provide control over fluid or hydraulically-operated functions. One example of such a valve is shown in GB-2201227-B. Typically, such a valve comprises a spool which is movable to and fro in a chamber and which carries shear seals urged against a respective wall of the chamber, the seals being disposed relative to fluid inlets and outlets to the chamber such that the spool can be moved between a first position, in which a fluid supply inlet is in communication with a functional outlet, and a second position in which the fluid supply inlet is cut off from the functional outlet and may if desired be in communication with a vent passage. It is known to control the movement of the spool between the first and second positions by means of solenoid valves which can supply a pulse of hydraulic pressure from a pilot supply to one or the other end of the spool.
It is also known, and shown by way of example in the aforementioned GB-2201227-B, to provide a safety piston which is spring loaded towards engagement with the spool in a sense to move the spool to a particular end position, particularly that associated with closure in which the fluid inlet is cut off from the functional outlet. The purpose of this safety piston, which is normally loaded by a mechanical spring, is to force the spool to the selected position in the event of failure of the pilot supply. It is necessary under normal operating conditions to supply pressure to the piston so as to move the position in the opposite sense, against the force of the spring and away from the spool, so that in the absence of failure of the pilot supply the safety piston is held away from the valve spool.